When high-speed and low voltage swing data transfer is needed, differential signaling (also commonly referred to as double ended signals), wherein signals are carried on two conductors and the signal is defined as the difference in the two signals. Differential signaling is perhaps the most robust and promising signaling concept. Current mode logic (CML), a design technique commonly used in high speed signaling applications such as communications chips and routers, uses differential signaling.
CML is widely used in high-speed applications due to its relatively low power consumption and low supply voltage when compared to other types of logic, such as emitter coupled logic (ECL). CML is also considerably faster than CMOS logic due to its lower voltage swings. CML also has an added advantage of the capability of being fabricated using CMOS fabrication technology.
When a signal needs to be transmitted off-chip, a signal driver commonly referred to as an off chip driver (OCD) may be used. An OCD may be used to provide sufficient driving current in order to transmit the signal on a transmission line. Certain OCDs may also provide voltage compatibility conversions. In a CML OCD, resistors are used to provide a necessary voltage drop that is necessary to the operation of the CML circuit.
When a signal is received over a transmission line, it is desired that the transmission line be properly terminated with resistors of a desired value so that the optimum signal transfer be achieved. In most situations, the resistors will have the same value (or approximately the same value) as the resistance seen by the signals being transmitted over the transmission line. These termination resistors are commonly referred to as on die termination (ODT) resistors.
Unless a particular application communicates in only one direction (either transmit or receive), a typical solution would be to have separate OCD and ODT circuits for each transmission line used in the application.
One disadvantage of the prior art is that through the use of separate OCD and ODT circuits, more resistors are used. In integrated circuits, it can be relatively difficult to produce resistors of a specific desired value, especially if the resistance of the resistors is large. This may lead to a more expensive integrated circuit.
A second disadvantage of the prior art is that integrated resistors (especially resistors with large resistances) may be physically large in size. Therefore, the use of a relatively large number of resistors may result in an integrated circuit that is physically large. Additionally, the presence of the large resistors may make it more difficult to route and place circuitry in the integrated circuit.